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141.
High-resolution benthic oxygen isotope and XRF (Fe and Ca) records from Site 1085 drilled in the Mid-Cape basin (ODP Leg 175) are used to investigate global climate changes during the Late Miocene in relation to Messinian geological events. The cyclic fluctuations of the time series at Site 1085 enable us to establish a reliable chronology for the time interval 7.3–4.7 Ma. Spectral analysis of the δ18O record indicates that the 41-kyr period of orbital obliquity dominates the Late Miocene record. A global climate record was extracted from the oxygen isotopic composition of benthic foraminifera. Both long- and short-term variabilities in the climate record are discussed in terms of sea-level and deep-water temperature changes. The time interval 7.3–6.25 Ma characterized by low-amplitude δ18O variations is followed by a period marked by maximum in the δ18O values (6.25–5.57 Ma). At about 5.56 Ma, a rapid decrease in δ18O values is documented that may reflect a warming of deep-water temperature associated with a global warming period. Comparison between the timing of the oceanic isotope events and the chronology of the Mediterranean Salinity Crisis suggest that global eustatic processes were not essential in the Mediterranean Salinity Crisis history. From our data, we infer that the global warmth documented in the Early/mid-Pliocene probably started during the Late Miocene (at 5.55 Ma). At the same time, the onset of evaporite deposition in the central basin of the Mediterranean Sea took place. Sharp changes in the sedimentation rates, mainly driven by terrigenous input at this site, are observed during the Messinian Stage. 相似文献
142.
A. M. Posadas F. Vidal J. Morales J. A. Pe a J. Iba ez F. Luzon 《Physics of the Earth and Planetary Interiors》1993,80(3-4):159-168
The three point method (TPM) has been successfully applied to several seismic series and has provided information about the spatial characteristics (azimuth and dip) of the fault planes activated in the rupture process. A new development of the TPM to determine temporal characteristics, is presented, to obtain the evolution of the fracturing process of an active fault system. For the analysis of the 158 microearthquakes and earthquakes that took place in the seismic series of Antequera in June 1989, the choice of a threshold magnitude (mu = 2.5) has permitted the events related to the most relevant fractures to be distinguished. Secondly, only the events between two concentric spheres (here named Spatial Crown) with respect to a given earthquake, have been used in order to avoid taking into account earthquakes that are too close to each other, together with the very distant events that have little relation to the event analysed. The Spatial Crown has permitted some clear results in the Antequera series, where we have found that the fracturing process began fundamentally with N 80° E planes and evolved to N 65° W planes. Finally, an error analysis enables an estimation of the uncertainty in the results to be obtained from the errors in the data. 相似文献
143.
Flix Gervais Kheireddine Rifai Philippe Plamondon Lütfü
zcan Franois Doucet Franois Vidal 《地学学报》2019,31(5):479-484
This contribution presents the first results of compositional tomography of a geological sample. The volume render of 6 × 8 × 1 mm3 was constructed by assembling 63 compositional maps acquired in 21 min (19.5 s/layer) by laser‐induced breakdown spectroscopy (LIBS), which determines the chemical composition of the analysed spot from the light emitted by a plasma produced by the laser. This technique is, therefore, able to directly reveal the 3D distribution of chemical elements in a sample. As an example, the spatial distribution and 3D geometry of visible gold in an ultramafic schist are presented. Inasmuch as this newly developed portable LIBS instrument is able to the rapidly characterize the 3D geometry of any geological materials, it has a high potential to be useful for the mining industry and for a wide range of geosciences, such as structural geology, petrology, sedimentology and economic geology. 相似文献
144.
Páramos are high‐altitudinal neotropical ecosystems located in the upper regions of the northern Andes. Their hydrology is characterized by an extraordinarily high run‐off ratio. One major contributing mechanism is thought to be fog occurrence, which is common in the páramos and occurs by the cooling of near‐surface moist air, as it is forced to higher elevations by topography. However, field‐based observations and quantification of this flux are rare. We present results of monitoring of occult precipitation, understood as the combination of fog and drizzle inputs, combined with meteorological and soil moisture monitoring for periods between 7 to 17 months in 6 sites distributed over 3 páramos catchments in Colombia: three sites in Romerales (Quindío), two in Chingaza (Cundinamarca), and one in Belmira (Antioquia). Occult precipitation inputs were measured with cylindrical fog gauges with a cover on top. We estimate occult precipitation inputs to add between 7% and 28% to rainfall inputs in the study sites. Our results also show that occult precipitation has a large temporal and spatial variability, both within one site and between sites, which make it difficult to upscale and quantify at a catchment scale. Nevertheless, occult precipitation can be important for downstream water supply given that these inputs are especially concentrated during periods with low rainfall. Lastly, we also find evidence for an increase in soil moisture related to occult precipitation during a dry period in Romerales páramo. 相似文献
145.
C. Bonadonna R. Cioni A. Costa T. Druitt J. Phillips L. Pioli D. Andronico A. Harris S. Scollo O. Bachmann G. Bagheri S. Biass F. Brogi K. Cashman L. Dominguez T. Dürig O. Galland G. Giordano M. Gudmundsson M. Hort A. Höskuldsson B. Houghton J.C. Komorowski U. Küppers G. Lacanna J.L. Le Pennec G. Macedonio M. Manga I. Manzella M. de’ Michieli Vitturi A. Neri M. Pistolesi M. Polacci M. Ripepe E. Rossi B. Scheu R. Sulpizio B. Tripoli S. Valade G. Valentine C. Vidal N. Wallenstein 《Bulletin of Volcanology》2016,78(11):84
146.
147.
M. B. Benito M. Navarro F. Vidal J. Gaspar-Escribano M. J. García-Rodríguez J. M. Martínez-Solares 《Bulletin of Earthquake Engineering》2010,8(4):739-766
A probabilistic seismic hazard assessment of Andalusia (Southern Spain) in terms of peak ground acceleration, PGA, and spectral
accelerations, SA(T), is presented in this paper. In contrast to most of the previous studies in the region, which were performed
for PGA, making use of Intensity-to-PGA relationships, hazard was here calculated in terms of magnitude, using published spectral
ground-motion models. Moreover, we considered different ground-motion models for the Atlantic sources, since the attenuation
of those motions seems to be slower, as evidenced in the case of the extensive macroseismic areas of earthquakes like those
occurred in the years 1755, 1969 and 2007. A comprehensive review of the seismic catalogue and of the seismogenic models proposed
for the region was carried out, including those for Northern Africa, which is part of the influence area. Hazard calculations
were performed following the Probabilistic Seismic Hazard Assessment (PSHA) methodology using a logic tree, which accounts
for six different seismic source zonings and five different ground-motion attenuation relationships. Hazard maps in terms
of PGA and SA (0.2 s) and SA (1 s) and coefficient of variation (COV) maps, for the 475-year return period were first obtained
in rock sites. A geotechnical classification and amplification factors were proposed and new hazard maps including local effects
were represented, showing PGA values ranging from 24 to 370 cm/s2 for the whole Andalusian territory, with the highest expected values (PGA > 300 cm/s2) in some parts of the Granada Province and in the town of Vélez Málaga. Lowest values (PGA < 50 cm/s2) correspond to some towns of the Huelva and Córdoba provinces. The inclusion of soil effects provides a more detailed picture
of the actual hazard the region is subjected to. 相似文献
148.
Basalts from the Marquesas Archipelago display significant variations according to magmatic type in 143Nd/144Nd (0.512710–0.512925) and 87Sr/86Sr (0.70288–0.70561) suggesting heterogeneities at various scales in the mantle source, with respectively the highest and lowest values in tholeiites compared to alkali basalts. This relationship is the reverse from that observed in the Hawaiian islands. Systematic indications of magma mixing are recognized from the relationships between trace element and isotopic ratios. Tholeiites from Ua Pou Island which have unradiogenic Sr (about 0.7028) plot close to basalts from Tubuai and St. Helena, i.e. distinctly below the main mantle trend in the Nd vs. Sr isotopic diagram. It is suggested that the source of these tholeiites is ancient subducted lithosphere which has suffered previous extraction of liquid with island arc tholeiite composition. The trace element and isotopic data of the basalts from the other Marquesas Islands imply the contamination of an equivalent source by an enriched component. This latter has trace element characteristics of the upper crust. 相似文献
149.
Tropical montane cloud forests (TMCF) receive additional (‘occult’) inputs of water from fog and wind-driven rain. Together with the concomitant reduction in evaporative losses, this typically leads to high soil moisture levels (often approaching saturation) that are likely to promote rapid subsurface flow via macropores. Although TMCF make up an estimated 6.6% of all remaining montane tropical forest and occur mostly in steep headwater areas that are protected in the expectation of reduced downstream flooding, TMCF hillslope hydrological functioning has rarely been studied. To better understand the hydrological response of a supra-wet TMCF (net precipitation up to 6535 mm y−1) on heterogeneously layered volcanic ash soils (Andosols), we examined temporal and spatial soil moisture dynamics and their contribution to shallow subsurface runoff and stormflow for a year (1 July 2003–30 June 2004) in a small headwater catchment on the Atlantic (windward) slope near Monteverde, NW Costa Rica. Particular attention was paid to the partitioning of water fluxes into lateral subsurface flow and vertical percolation. The presence of a gravelly layer (C-horizon) at ~25 cm depth of very high hydraulic conductivity (geometric mean: 502 mm h−1) intercalated between two layers of much lower conductivity (7.5 and 15.7 mm h−1 above and below, respectively), controlled both surface infiltration and delayed vertical water movement deeper into the soil profile. Soil water fluxes during rainfall were dominated by rapid lateral flow in the gravelly layer, particularly at high soil moisture levels. In turn, this lateral subsurface flow controlled the magnitude and timing of stormflow from the catchment. Stormflow amount increased rapidly once topsoil moisture content exceeded a threshold value of ~0.58 cm3 cm−3. Responses were not affected appreciably by rainfall intensity because soil hydraulic conductivities across the profile largely exceeded prevailing rainfall intensities. 相似文献
150.
Vidal Javier Marcé Rafael Serra Teresa Colomer Jordi Rueda Francisco Casamitjana Xavier 《Aquatic Sciences - Research Across Boundaries》2012,74(2):315-327
The local response of the phytoplankton community to river inflow processes was investigated with modeling and field analyses
in a long and narrow, stratified reservoir in mid-summer. The river water had high concentrations of phosphorus and nitrogen
(ammonium and nitrate) and temperature had large variations at diurnal scales. As a consequence of the large variation in
river temperature, the level of neutral buoyancy (the depth where the river water spreads laterally in the reservoir) oscillated
between the surface (overflows) during the day, and the depth of the metalimnion (interflows) during the night. The reservoir
remained strongly stratified, which favoured the presence of cyanobacteria. It is shown that under these conditions, nutrient-rich
river water injected during overflows into the surface layers promoted the occurrence of localized algal blooms in the zones
where the overflow mixed with the quiescent water of the reservoir. A series of hydrodynamic simulations of the reservoir
were conducted both with synthetic and realistic forcing to assess the importance of river temperatures and wind-driven hydrodynamics
for algal blooms. The simulations confirmed that the river inflow was the main forcing mechanism generating the localized
bloom. 相似文献